The present invention relates to an optical waveguide or, more particularly, to an optical waveguide of fused silica glass with a remarkably low propagation loss to be suitable for use as an optical branch, which can be manufactured at low costs with good productivity.
Optical waveguide is an element in an optical fiber circuit used for coupling of lines or branching of lines. An optical waveguide has an integral structure, in which a transparent undercladding layer is formed on a substrate, such as a fused silica glass plate and a semiconductor silicon wafer, and a core line having a refractive index larger than that of the undercladding layer is formed on the undercladding layer while the undercladding layer and the core line thereon are altogether coated with an also transparent overcladding layer having a refractive index smaller than that of the core line. When the substrate is made from a transparent material such as fused silica glass, the substrate serves also as the undercladding layer so that no separate undercladding layer is required. Since optical fibers currently used for optical communication are predominantly made from fused silica glass, a preferable material for the optical waveguide is also fused silica glass in view of the good matching performance with the optical fibers.
A typical process for the preparation of an optical waveguide of fused silica glass described above is as follows by utilizing the flame deposition method and reactive ion etching method. Thus, a first porous layer of fine silica glass particles is formed by the flame deposition method on a substrate such as a silicon wafer and the porous layer is vitrified by heating at a high temperature in an electric furnace to form a transparent undercladding layer. Thereafter, a second porous layer of fine silica glass particles having a larger refractive index than the first is formed by the flame deposition method on the undercladding layer followed by vitrification into a transparent silica glass layer which is then subjected to the process of reactive ion etching to form an elongated core line having a square or rectangular cross section. Further, a third porous layer of fine silica glass particles having a smaller refractive index than the core line is formed by the flame deposition method on the core line and on the undercladding layer to form a covering layer which is vitrified to give a transparent overcladding layer which integrally covers the core line and the undercladding layer (see, for example, Optronics, 1988, No. 8, page 85). Sometimes, a protective top coating layer is formed on the overcladding layer.
Starting from a fused silica glass plate as the substrate, which serves also as the undercladding layer as is mentioned above, a method of electron-beam vapor deposition is proposed for forming, on the substrate, a porous silica glass layer which is vitrified and shaped into a core line of square or rectangular cross section by the reactive ion etching method followed by the formation of an overcladding layer in the same manner as above (see, for example, IEEE, 1991, page 483).
Though different in respect of the procedures for the formation of the undercladding layer and the core line, the above described two methods commonly utilize the flame deposition method for the formation of the overcladding layer.
In place of the procedure in the above described prior art method in which the silica glass layers are formed by the vitrification of a porous layer of silica particles deposited by the flame deposition method or electron-beam vapor deposition method, proposals have been made for the use of a silicon-containing polymeric resin as a precursor of silica according to which a layer of a silicone polymer is exposed to plasma in an atmosphere of oxygen (see Journal of Vacuum Science and Technology, volume 29, No. 17, page 1197) or heated in oxygen or in air (B. G. Bagley, Better Ceramics through Chemistry, page 287) to be converted into a layer of silica glass. Further, Japanese Patent Kokai 5-88036 teaches that each of the waveguide layers and core line can be formed from a glassy material derived from a heat-resistant organopolysiloxane.
Though advantageous in respect of the small propagation loss, the optical waveguide, for which the flame deposition method or the electron-beam vapor deposition method is employed for the formation of the core line, has several problems described below.
Namely, the flame deposition method, which is advantageous relative to the control of the refractive index of the vitrified layer because control of the refractive index can be performed by merely adding a dopant element to the starting material for the silica particles, unavoidably has a difficulty in the controllability of the layer thickness and uniformity in the thickness distribution of the layer so that satisfactory uniformity in the thickness of each layer can be ensured only by conducting lapping and polishing of each of the layers after vitrification.
The electron-beam vapor deposition method, on the other hand, is advantageous in the controllability of the layer thickness and uniformity in the layer thickness requiring no lapping treatment of each of the vitrified layers. A problem in the electron-beam vapor deposition method is in the relatively low velocity of silica deposition so that it is hardly practicable to apply this method to the formation of all of the three layers for the undercladding layer, core line and overcladding layer.
The method using an organopolysiloxane as a precursor of silica glass for the layers in an optical waveguide is indeed advantageous in respects of the controllability of refractive index of the silica glass, controllability of the layer thickness, uniformity of layer thickness and productivity. When the core line is formed by the reactive ion etching method or photolithographic method of the vitrified layer obtained from such an organopolysiloxane polymer as is disclosed in Japanese Patent Kokai 5-88036, the propagation loss of the optical waveguide cannot be small enough.